std::less (3) - Linux Manuals

std::less: std::less

NAME

std::less - std::less

Synopsis

Defined in header <functional>
template< class T > (until C++14)
struct less;
template< class T = void > (since C++14)
struct less;

Function object for performing comparisons. Unless specialized, invokes operator< on type T.

Specializations

A specialization of std::less for any pointer type yields a strict total order, even if the built-in operator< does not. The strict total order is consistent among specializations of std::less, std::greater, std::less_equal, and std::greater_equal for that pointer type, and is also consistent with the partial order imposed by the corresponding built-in operators (<, >, <= and >=).

If the function call operator of the specialization std::less<void> calls a built-in operator comparing pointers, it yields a strict total order even if the built-in operator< does not. This strict total order is consistent among the specializations std::less<void>, std::greater<void>, std::less_equal<void>, and std::greater_equal<void>, and is also consistent with the partial order imposed by the corresponding built-in operators. (since C++14)

The standard library provides a specialization of std::less when T is not specified, which leaves the parameter types and return type to be deduced.

           function object implementing x < y deducing argument and return types (since C++14)
less<void> (class template specialization)

Member types

Type Definition (until C++20)
result_type(deprecated in C++17) bool
first_argument_type(deprecated in C++17) T
second_argument_type(deprecated in C++17) T

Member functions

           checks whether the first argument is less than the second
operator() (public member function)

 std::less::operator()

bool operator()( const T& lhs, const T& rhs ) const; (until C++14)
constexpr bool operator()( const T& lhs, const T& rhs ) const; (since C++14)

Checks whether lhs is less than rhs.

Parameters

lhs, rhs - values to compare

Return value

true if lhs < rhs, false otherwise.

Exceptions

(none)

Possible implementation

  constexpr bool operator()(const T &lhs, const T &rhs) const
  {
      return lhs < rhs;
  }

Example

// Run this code

  #include <functional>
  #include <iostream>

  template <typename A, typename B, typename U = std::less<>>
  bool f(A a, B b, U u = U())
  {
      return u(a, b);
  }

  int main()
  {
      std::cout << std::boolalpha;
      std::cout << f(5, 20) << '\n';
      std::cout << f(100, 10) << '\n';
  }

Output:

  true
  false